Stainless steel and method



May 2, 1950 G. N.v GOLLER STAINLESS STEEL AND METHOD Filed Dec. 14, 1949TBXOIN N30 83d @0265 N. Gouf/z.

mw f4 f '-11.3 flllmnm! Patented May 2, 1950 UNITED STATES PATENT OFFICESTAINLESS STEEL AND METHOD George N. Goller, Baltimore, Md., assignor4to Armco Steel Corporation, a corporation of Ohio Application December14,1949, Serial No. 132,832 7 Claims.A (Cl. 14S-21.54)

My application for patent is a continuationin-part of my copendingapplication, Serial No. 695,216 filed September 6, 1946, and entitledStainless steel and method and also it is a continuation-in-part of mycopending application, Serial No. 695,217 of the same date and title,and the invention relates to chromium-nickel stainless steels oflow-carbon content, more particularly to a method for conditioning thesteels to the hardenable or hardened state, and to the resultingproducts and manufactures themselves.

An object of my invention is the provision of chromium-nickel stainlesssteels which are amenable to hardening by heat treatment from a soft,formable and machinable condition.

Another object is the provision of chromiumnickel stainless steels soproportioned in composition as to be consistently and reliably producedto analysis specification and of such composition as to be amenable toannealing, and working and fabricating in the annealed condition, andthereafter to hardening treatment Without, however, being undulysensitive to cold weather in the prefabricated condition.

A further object of my invention is the provision of a simple, practicaland reliable method for conditioning chromium-nickel stainless steels toa workable state suited for such operations as cold working, forming,stamping, punching, spinning, drawing, bending, heading, and machining,and to the hardened state by heat exchange treatment at temperaturessufficiently low to avoid substantial warping and the formation of heatscale.

A still further object of my invention is the provision of hardenedchromium-nickel stainless steels, as for example cold worked andhardened articles and products, which are characterized by excellentphysical properties including high yield strength and high ultimatestrength.

Other objects in part will be obvious and in part pointed outhereinafter.

The invention accordingly consists in the combination of elements,composition of ingredients, and in the several method steps and therelation of each of the same to one or more of the others as describedherein, and in the resulting articles and products had thereby, thescope of the application of which is indicated in the following claims.

The single figure of the accompanying drawing graphically representsproportions of chromium and nickel which are employed in the compositionof my stainless alloy steel, as will be pointed out more fullyhereinafter.

As conducive to a clearer understanding of certain features of myinvention, it may be noted at this point that the better knownchromiumnickel stainless steels, as for example the 18% chromium-8%nickel steels, are stably austenitic at room temperatures and do notappreciably respond to hardening by heat treatment. They do attainhardness by cold working but, in actual practice, there are sharplimitations upon the possible use of this property to achieve desiredhardness. The products so hardened are under strain, and the customaryannealing treatment to relieve the strain tends to reduce the hardness.There are many valuable properties of the steels, for instance coldworkability as by rolling, swaging, cold heading and drawing,machinability, and, as well, many additional features such as excellentresistance to corrosion, which place the steels in very considerabledemand.

In accordance with the general definition of stainless steels, thechromium-nickel varieties frequently include additions of such elementsas copper, manganese, silicon, cobalt, molybdenum, tungsten, vanadium,titanium, columbium, and sulphur, for special purposes. Some few of thechromium-nickel stainless steels have been known to respond to hardeningby heat treatment, this by virtueof the addition of titanium or colum-'bium in well studied proportionment with other elements present coupledwith a critical form of heat treatment to effect precipitationhardening. Columbium and titanium, however, are relatively expensivematerials. The chromium-nickeltitanium or chromium-nickel-columbiumstainless steels, moreover, usually contain stress laden ferrite as anessential to hardenability from Athe annealed condition. When the steelsare produced to such composition as to present a comparatively soft,substantially fully Iaustenitic structure at about room temperaturesafter annealing, they are more amenable to cold working and formingoperations, but hardenability by heat treatment is usually sacrificed.

The production and treatment of certain grades of stainless steelsheretofore has involved the introduction of aluminum into the steel meltfor serving as a deoxidizing material, and also as an alloying elementto develop an aluminumcontaining oxide film on the metal which isresistant to heat and corrosion. The alloying of aluminum with suitablequantities of other included elements of the steel is known to impart`high temperature properties which, depending upon degree, havejustified the production from the metal of such articles as heat enginevalves,

aooaose tion which is retainable down to substantially below ordinaryroom temperature. This heating is in the nature of an annealingtreatment. To achieve the annealing, I usually adjust the metal totemperature and maintain or hold the same at temperature as in aconventional heat-treating furnace for a suitable period of time. 'I'heholding time as applied to the steel is not too critical. About one-halfhour usually is quite satisfactory from the standpoint of economy and ofensuring adequate solubility of the aluminum. I then quench the annealedmetal as in air, oil or water, preferably to around room temperature. Asa result of the annealing and quenching, the stainless steel displays asoft, substantially fully austenitic structure, is ductile, has gooddirectional properties, hardnesses usually below about Rockwell B-92,and readily lends itself to forming, machining, and the like. 1

I '11nd, as briefly noted above, that with the quenching, however,certain internal stresses apparently are set up in the steel whichpermit the transformation from austenite to martensite upon subsequentsevere cooling'. Thus, in shipping the steel during unusually coldwinter weather, I note that a hardening occurs which in many cases ismost objectionable. Under unusual conditions, the hardness had mayamount to as much as Rockwell C-30. This hardening notably increases thedifficulties of the forming, machining or like operations.

In accordance with my invention, I subject the steel, following theannealing and quenching steps, to a stabilizing treatment, this byheating the steel to a temperature of some 600 F. to 1100" F. for ashort period of time. Ordinarily, heating at a temperature of 900 F. forabout one hour is found to give good results. Where the lowertemperatures are employed, however, a longer period of time is used andconversely where the higher temperature is used less time of treatmentis required. This treatment leaves the steel with substantially nochange in hardness or other physical properties.

My stainless steel at any time subsequent t from the standpoints ofyield, in tension and compression coupled with toughness and corrosionresistance. I provide cold-headed bolts and screws, as for example thoseeventually to include hardened Shanks; surgical instruments and dentaltools; valve and valve seats; die blocks; and the like. In all of thesearticles of ultimate use, I take advantage of the excellent workabilityand formability of the soft annealed metal through such operations ascold-forming, upsetting, drawing, spinning, stamping, punching,machining, and other steps consistent with the properties of the metal.Although I prefer to conduct the working and fabricating operationsprior to any -hardeningv treatment, I may delay part or all of theforming and fabricating operations until after the achievement of apreliminary hardening 'pre-treatment now to be described.

To preliminarily harden the annealed and stabilized chromiumnickelaluminum stainless steel, as after the forming and fabricatingsteps illustratively conducted before undertaking the hardeningtreatment, I subject the steel toa heat exchange treatment consistingeither of heating the steel to substantial tem-peratures or of coolingit to very low temperatures. For the one, I reheateit, illustratively inthe same heat-treating furnace employed for the annealing, this time upto within the approximate temperature range of 1200 F. to 1600* F.,preferably up to about 1400 F., and there hold the metal to desiredtemperature or temperatures for approximately 1A hour or more. Duringthe holding operation, as for example with the steel at around 1400 F.,there results a precipitation of carbides, possibly some precipitationof aluminum compounds, and a raising of the transformation point of theaustenitic matrix. The steel, upon cooling, consequently transforms toinclude a soft martensite-llke constituent. The transformation occurs atquite low temperature such as between about F. and 225 F. Thus, at thecompletion of the reheating stay, I quench the steel as in air, oil orwater with the result that transformation does occur above roomtemperature to, achieve a partial hardening of the metal, or of thearticles made thereof as described hereinbefore. Hardnesses after thistransformation vary in or near the range of Rockwell C-24 to C-28depending upon the particular composition and the conditions oftreatment within the limits noted.

Where I employ a low temperature heat exchange treatment, I cold-treatthe metal, as in fabricated condition, at temperatures of about 30 F. to100 F. or lower, this preferably being in a suitable coolingcompartment, or the like. A satisfactory temperature is 60 F. It will beunderstood that because of the extremely low temperature required, mysteel does not preliminarily harden to any substantial extent duringshipment even under the most severe cold weather conditions.

In preliminarily hardening the steeleby cold treatment, a holding timeof about 1/2 hour 0r more at cooling temperatures within the coolingrange noted is preferred to effect phase transformation and thepreliminary hardening. The cooling medium illustratively is dry ice inacetone. Hardnesses of the chromium-nickel stainless steel after thistransformation vary in or near the range of Rockwell C-32 to C-36depending upon the particular composition and conditions of treatmentwithin the limits mentioned.

The chromium-nickel-aluminum stainless steel in the preliminarilyhardened condition whether by heat treatment or by cold treatment,retains a reasonable amount of workability and/formability,illustratively cold rolling or drawing properties, and the steel ismachinable and may be fabricated as by cutting, punching, drilling, andthe like. It is for this reason and also the possi'- bility of furtherhardening the metal without phase transformation and changes indimensions of the metal accompanying transformation that I frequentlyput off all or part of certain forming and fabricating operations to beaccomplished on the same until after the preliminary hardeningtreatment, and then produce any of a wide variety of articles andproducts, as to substantially iinished dimensions, of the partiallyhardened steel.

When transformation has been achieved by the heat exchange treatment asdescribed, and appropriate working and forming operations effected wheredesired, the alloy steel then is ready for positive precipitationhardening treatment. In performing this operation, representing afurther axiomatic4 part army conditioning treatinent, I neat the steelwithin a temperature range of about 750 F. to approximately 1000 F.,preferably at /about 900 F.,.and there hold vthe same at temperature forabout one hour. The time of holding under treatment, however, may varyfrom approximaten ly one-half hour for the high temperature of 1000 F.to some ve hours or more for the low temperature of 750 F., this withsatisfactory re- 1 sults. About one hour treatment at 900 F. ispreferred, however. part a material gain in hardness to the metal. Thishardness, I ilnd, `upon quenching the metal as in air, oil or water fromthe hardening temperatures, comes within the approximate range of C40 toC-46 Rockwell. A precipitated aluminum-rich phase is instrumental to thehardness. This I believe to be an aluminum-nickel compound.

In the precipitation hardened condition which results from the heatingat 750 F. to 1000 F., the so-conditioned steel or steel products areIvery strong both in tension and compression, have The treatment servesto imi high yield strength, good directional qualities,

and are quite resistant to corrosion. The steel emerges from thepreliminary hardening treatment and precipitation hardening heattreatment substantially free of heat scale and unwarped by heat at thelow temperatures employed.

As illustrative of the stainless steel and method l 2. By combined coldtreatment and heat treatment: i f

F. Samples ol E iient No. `E'nnsil-Jtlrllnctsodof-33' T110 PrecipitationF 20mm hardened 900 F.

' l hr. air cooled C35 C43 C34 C42 C34 C41 TABLE IV Hardness ofunstabilized steels as compared to stabilized steels G. Samples of A(un- H. Samples of B Heut No stabilized) subjected (Stabilized)subjected 32 F. 20 mins. 32 F. 20 mins.

C35 B90 C33 B79 C B75 of my invention, three samples of stainless steeli of composition noted in Table I were subjected to various conditionsof heat-treatment with the I results as reported below in Tables 1I andIII.

TABLE I Composition of stainless steels testedv HentNo. c Mn 1 s Isi crNi A1 .oso .6i 62o .oi5 .4a 16.54 1.01 1.o:

051 .11 .624 .oio 55 16.11 1.06 .as

TABLEH stabilized following annealing i Hardness of annealed samples andof samples It win be seen that with the stabilizing treatment there issubstantially no change in hardness.

TABLE III B given heat exchange treatment and then precipttationhardened l. By combined double heat-treatment:

C- Steps?. tit;- D-Trmpla 0 Y re a en precipi ion Heat No 3 hrs. waterhardened 900 F.

quenched l hr. air cooled c2c C43 C C42 C C43 Hardness of annealed andstabilized samples of The superior working and forming characteristicsof the stabilized steels are evident.

The close correlation of aluminum, a comparatively cheap material, withother elements in the steel accordingly enables a commercially valuablehardening effect, which becomes active with proper treatment of themetal from a soft workable or Worked condition. It will further beappreciated that the aluminum contributes to heat resistance and to theprevention of heat scale formation as during the low temperatureprecipitation hardening heat in the vicinity of '750 F. to 1000 F. andin the resulting precipitation hardened, corrosion resistant products.

' Thus, it will be seen that in this invention there is provided animproved precipitation hardening stainless steel and method oi treatingthe same in which the various objects hereinbei'ore. noted together withmany thoroughly practical advantages are successfully achieved. It willbe ,Y

seen that the method makes possible the provision of wrought or castchromium-nickelaluminum stainless steel subjected to any of a number offorming, machining or fabricating operations and effectively andreliably hardened by heat exchange either at substantial temperatures orat low temperatures.` Likewise, it will be seen that my method isreadily practiced, and enables the production of chromium-nickel gradestainless steel oi?v hardened quality with a minimum of any suchtreatment as pickling, and otherwise is quite suitable for commercialuse. It will further be noted that the steels provided have gooddirectional properties, are quite soft and ductile in the annealedcondition and in the annealed and stabilized condition, and are,yhardenable to a condition of reasonable ductility and high yield andultimate strengths.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth, it is tobe understood that all matter described herein is to be interpreted asillustrative and not as a limitation.

I claim:

1. In a method of hardening chromiumnickel alloy stainless steel, theart which includes providing a steel containing chromium and nickel insubstantial accord with area ABCD of the accompanying diagram, aluminumin amounts between about 0.50% to 2.50%, and

from incidental amounts up to approximately 2.0% silicon, manganese fromincidental amounts up to approximately 8.0%, and carbon between about0.02% and 0.12%, and the remainder substantially all iron; thenannealing and quenching the steel to provide an austeniticaluminumsoluble structure transformable below usual room temperature;heating the steel to a temperature of about 600 F. to 1100 F. tostabilize the same; subjecting said steel to heat exchange pre-treatmentat such temperatures and for long enough time to achieve transformation;and heating the transformed alloy at a sufllciently high temperature andfor a sufiicient period of time to precipitate an aluminum compound andobtain 'a substantial increase in the hardness of the steel.

2. In a method of hardening chromium-nickel alloy stainless steel, theart which includes providing a steel containing about 16.5% to 17.5%chromium, with about 6.75% to 7.5% nickel, aluminum in amounts betweenabout 0.60% to 1.1%, and about 0.06% to 0.08% carbon, and the remaindersubstantially all iron; then annealing and quenching the steel toprovide an austenitic aluminum-soluble structure transformable belowusual room temperature; heating the steel to a temperature of 600 F. to1100 F. to stabilize the same; subjecting said steel to heat exchangepretreatment consisting of cold treatment below 4about 301 F. forsufficient time to achieve 3. In the production of hardened cliromiumu.

nickel alloy stainless steel articles and products, the art whichincludes providing a steel containing about 16.5% to 17.5% chromium,with about 6.75% to 7.5% nickel, aluminum in amounts between about 0.60%and 1.1%, and about 0.06% to 0.08% carbon, and the remaindersubstantially all iron; annealing saidv steel at approximately 1800 F.to 2000 F.` and quenching thev same to provide an austeniticaluminum-soluble structure transformable below usual room-temperature;stabilizing the steel by treatment at 600 F. to 1100 F.; fabricatingarticles and products of the austenitic stabilized steel; subjectingsaid fabricated articles and products to heat exchange pre-treatment at1200 F. to 1600 F. for suicient time to achieve transformation; andyheating the transformed articles or products within the approximatetemperature range of 750 F. to 1000 F. for a sufllcient period of timeto precipitate an aluminum compound and obtain a substantial increase inhardness of the articles and products.

4. An annealed and stabilized chromiumnickel stainless steel susceptibleto aluminumprecipitation-hardening by heat exchange treatment from theannealed and stabilized condition involving a preliminary heat-exchangetreatment followed by linal precipitation hardening heat treatment, saidsteel containing the chromium-like components aluminum in the amount ofabout 0.50% to 2.50%, silicon from incidental amounts up to about 2.0%based on total content of the steel, and the remainder substantially allchromium, said components in total meeting the terms for chromium ofarea ABCD of the accompanying diagram, said chromium and silicon servingas a substitute for chromium on about a l to 1 ratio; the nickel-likecomponents carbon in the amount of about 0.02% to 0.12%, manganese fromincidental amounts up to 8.0%, and the remainder substantially allnickel in actual amount not less than about 3.5%, said nickel-likecomponents in total meeting the terms for nickel of area ABCD of theaccompanying diagram. and said carbon and manganese serving as asubstitute for nickel on the ratios of about 1/iui to 1/zm part carbonto 1, and approximately 2 parts manganese to 1; and the remaindersubstantially all iron.

5. An annealed and stabilized chromiumnickel stainless steel ofsubstantially fully'austenitic structure at usual room temperatures,said steel containing about 16.5% to 17.5% chromium, about 6.75% to 7.5%nickel, aluminum between about 0.60% and 1.1%, about 0.06% to 0.08%carbon, and the remainder substantially all iron, and the steel beingsusceptible to hardening by heat exchange treatment involvingpretreatment at 1200 F. to 1600 F., after the annealing and stabilizingfollowed lby linal precipitation hardening.

6. A chromium-nickel stainless steel aluminum-precipitation-hardened byheat exchange treatment involving preliminary treatment below about 30F. from the annealed and stabilized condition followed byprecipitation-hardening final heat treatment, said steel containingchromium and nickel in amounts substantially in accordance with areaABCD in the accompanying diagram, carbon between about 0.02% to 0.12%,about 0.50% to 2.50% aluminum, from incidental amounts up toapproximately 8.0% manganese, from incidental amounts up to about 2.0%silicon, and the remainder substantially all iron, said aluminum beingprecipitated as aluminum compound within the matrix of the steel to sivesubstantial hardness.

7. In the production of a stabilized precipitation-hardenablechromium-nickel alloy stainless steel, the art which includes providinga steel containing the chromium-like components aluminum in the amountof about 0.05% to 2.50%. silicon from incidental amounts up to about2.0%

based on total content of the steel, and the remainder substantially allchromium, said components in total meeting the terms for chromium ofarea ABCD of the accompanying diagram, said chromium and silicon servingas a substitute for chromium on about a 1 to 1 ratio; the nickel-likecomponents carbon in the amount of about 0.02%

to 0.12%, manganese from incidental amounts up to 8.0%, and theremainder substantially all nickel in actual amount not less than about3.5%, said nickel-like components in total meeting the terms fornickelof area ABCD of the No references cited.

1. IN A METHOD OF HARDENING CHROMIUMNICKEL ALLOY STAINLESS STEEL, THEART WHICH INCLUDES PROVIDING A STEEL CONTAINING CHROMIUM AND ANICKEL INSUBSTANTIAL ACCORD WITH AREA ABCD OF THE ACCOMPANYING DIAGRAM, ALUMINUMIN AMOUNTS BETWEEN ABOUT 0.50% TO 2.50%, AND FROM INCIDENTAL AMOUNTS UPTO APPROXIMATELY 2.0% SILICON, MANGANESE FROM INCIDENTAL AMOUNTS UP TOAPPROXIMATELY 8.0%, AND CARBON BETWEEN ABOUT 0.02% AND 0.12%, AND THEREMAINDER SUBSTANTIALLY ALL IRON; THEN ANNEALING AND QUENCHING THE STEELTO PROVIDE AN AUSTENITIC ALUMINUMSOLUBLE STRUCTURE TRANSFORMABLE BELOWUSUAL ROOM TEMPERATURE; HEATING THE STEEL TO A TEMPERATURE OF ABOUT600*F. TO 1100*F. TO STABILIZE THE SAME; SUBJECTING SAID STEEL TO HEATEXCHANGE PRE-TREATMENT AT SUCH TEMPERATURES AND FOR LONG ENOUGH TIME TOACHIEVE TRANSFORMATION; AND HEATING THE TRANSFORMED ALLOY AT ASUFFICIENTLY HIGH TEMPERATURE AND FOR A SUFFICIENT PERIOD OF TIME TOPRECIPITATE AN ALUMINUM COMPOUND AND OBTAIN A SUBSTANTIAL INCREASE INTHE HARDNESS OF THE STEEL.
 2. IN A METHOD OF HARDENING CHROMIUM-NICKELALLOY STAINLESS STEEL, THE ART WHICH INCLUDES PROVIDING A STEELCONTAINING ABOUT 16.5% TO 17.5% CHROMIUM, WITH ABOUT 6.75% TO 7.5%NICKEL, ALUMINUM IN AMOUNTSA BETWEEN ABOUT 0.60% TO 1.1%, AND ABOUT0.06% TO 0.08% CARBON, AND THE REMAINDER SUBSTANTIALLY ALL IRON; THENANNEALING AND QUENCHING THE STEEL TO PROVIDE AN AUSTENITICALUMINUM-SOLUBLE STRUCTURE TRANSFORMABLE BELOW USUAL ROOM TEMPERATURE;HEATING THE STEEL TO A TEMPERATURE OF 600*F. TO 1100*F. TO STABILIZE THESAME; SUBJECTING SAID STEEL TO HEAT EXCHANGE PRETREATMENT CONSISTING OFCOLD TREATMENT BELOW ABOUT - 30*F. FOR SUFFICIENT TIME TO ACHIEVETRANSFORMATION; AND FINALLY HEATING THE TRANSFORMED ALLOY AT ASUFFICIENTLY HIGH TEMPERATURE FOR A SUFFICIENT PERIOD OF TIME TOPRECIPITATE AN ALIMINUM COMPOUND AND OBTAIN A SUBSTANTIAL INCREASE INHARDNESS OF THE STEEL.